In recent years, portable electronic equipment is continually being downsized, while simultaneously being more highly advanced. Typically, such portable electronic equipment utilize a small secondary battery (a storage battery) as a driving power source. Typically, the secondary battery includes a lithium ion battery and a nickel-metal hydride battery.
The nickel-metal hydride battery is a battery of high energy density made of a positive electrode of a porous nickel substrate coated with nickel hydroxide paste and a negative electrode of a hydrogen absorbing alloy. With the progress in size reduction and performance of the portable electronic equipment, enhancement of the storage battery performance is constantly being required. The enhancement of the storage battery performance means an increase in output and battery capacity.
As means of increasing the battery capacity, there are methods of {circle around (1)} increasing the size (volume) of the storage battery, {circle around (2)} improving the performance of an active material (hydrogen absorbing alloy), {circle around (3)} increasing the filling amount of the active material and {circle around (4)} reducing the volume of battery components.
As measures for the above-described methods of {circle around (3)} increasing the filling amount of the active material and {circle around (4)} reducing the volume of the battery components, negative electrodes improved in various ways have been proposed (for example, see Japanese Examined Patent Publication No. SHO 58-46827, Japanese Laid-Open Patent Publication No. SHO 53-33332, Japanese Laid-Open Patent Publication No. SHO 61-163569 and Japanese Laid-Open Patent Publication No. HEI 10-188994).
The negative electrode of the nickel-metal hydride battery is formed by applying the active material in the slurry form onto both surfaces of porous metal foil (a core material), drying at about 100° C., followed by press-bonding using rollers. This production method is adopted to increase the filling density of the active material, and at the same time, to enhance adhesion (adherability) between the active material and the porous metal foil.
So far, nickel-plated steel foil having a plurality of small apertures and a thickness of about 60-80 μm has been used as the above-described porous metal foil for the core material. The nickel-plated steel foil is formed by making a steel plate into steel foil of 50-70 μm thick by cold rolling, boring therein a plurality of small apertures using a press boring apparatus (a punching machine) and nickel plating on the surface thereof (see Japanese Laid-Open Patent Publication No. SHO 61-163569).
For the purpose of increasing the energy density of the battery, there has been proposed an alkaline storage battery using, as the core material of the negative electrode, porous nickel-plated steel foil having the whole thickness (total thickness of the base steel foil and the plated layer) of 20-50 μm thick or the porous steel foil subjected to heat treatment to give tensile strength and malleability (see Japanese Laid-Open Patent Publication No. HEI 10-188994). The harder the porous steel foil is, the better the aperture shape becomes in boring (punching) the apertures, and the punching can be carried out at high speed. However, if the active material is applied and press-bonded after the nickel plating, only a small amount of the active material is adhered to the foil (i.e., the adherability of the active material becomes poor). Therefore, the punching is carried out after rolling and then the nickel-plating is performed, followed by softening or annealing.
In order to form the steel foil which serves as the base material of the above-described porous nickel-plated steel foil having the whole thickness of 20-50 μm, a large amount of energy is required for the rolling. Further, cost increase due to yield reduction is inevitable. If the porous nickel-plated steel foil is subjected to the softening or annealing, the adhesion (adherability) of the active material increases, but the tensile strength decreases. Therefore, the foil may possibly be torn in a press-bonding step after the application of the active material or during transfer in a process of assembling the foil into the battery.
On the other hand, an electrolytic deposition method is the most rational method for manufacturing the porous metal foil as thin as 35 μm or less.
FIG. 1 is a diagram illustrating the principle of the electrolytic deposition method. Hereinafter, explanation is given by way of an example of the production of nickel foil. According to this method, an electrodeposition drum 2 to be a cathode and a semi-circle anode 3 made of nickel are arranged at a predetermined interval, an electrolyte 4 is flown therebetween and electric current is passed while rotating the electrodeposition drum 2. Then, nickel is electrodeposited lightly onto the surface of the electrodeposition drum 2, which is peeled off to collect as nickel foil 1.
The above-described electrodeposition drum 2 is made of a material that is insoluble to the electrolyte 4 such as titanium. As shown in FIG. 2 (a partial development of the drum surface), a plurality of holes 2-2 of 1-2 mm diameter are provided on the periphery of a base material 2-1. Further, as shown in FIG. 6A, the holes 2-2 are filled with an insulating resin 2-3. Therefore, nickel is not electrodeposited on the holes 2-2 and the resulting nickel foil 1 becomes porous.
According to the above-described method, in principle, the porous metal foil is formed continuously. However, there are significant problems to be solved before placing this technique into practical use. One of the problems is that the resin 2-3 adheres to the deposited metal foil (nickel foil) 1 when peeling the foil off the electrodeposition drum 2 and separates from the hole 2-2. Then, electrolytic deposition occurs in the hole 2-2 (on the inner surface thereof) where metal must not be electrodeposited, which makes impossible to form the porous metal foil. Another problem is that the metal foil (nickel foil) 1 is apt to be torn when peeling. Especially in the case of ultra-thin foil or foil having a large aperture rate, the peeling from the electrodeposition drum 2 cannot be carried out properly, which may often tear the foil.
With respect to the above-described separation of the resin, Japanese Laid-Open Patent Publication No. HEI 8-100288 discloses a solution. In this method, cold water is poured onto the drum surface immediately after the drum is taken out of the electrolyte to reduce an adhesion force between the metal foil and the drum (resin). However, the effect is doubtful. Even if the method is effective, it is disadvantageous in that the composition and temperature of the electrolyte may be changed by the cooling water and the conditions for the electrolytic deposition may vary.